Recently, as multimedia applications have been further broadened, it has become more and more commonplace to convert information on every type of medium, including image, audio and text data, for example, into digital data and process it collectively. Among other things, the size of the digital image data (e.g., digital moving picture data, in particular) is so huge that an encoding technique for encoding the moving picture data highly efficiently is required to store and transmit that data. As exemplary encoding techniques that were developed for the purpose of encoding a moving picture, encoding schemes as like MPEG-1, 2 and 4 defined by International Organization for Standardization (ISO), are known.
According to any of these encoding schemes, before image data is subjected to orthogonal transformation and quantization, a quantization matrix is specified. The respective elements of the quantization matrix are provided for their associated frequency components of an orthogonal transformation coefficient. And the orthogonal transformation coefficient is quantized with the values of those elements. The bit allocation property in a frequency space can be controlled according to how to define the value of the quantization matrix. That is why the quantization matrix should represent one of the features of the quantization processing (which will be referred to herein as “quantization characteristic”).
Two different quantization matrices can be defined for an intra coding process and an inter-picture (or non-intra) predictive coding process, respectively. As a result, two different frequency bit allocations can be made according to the method of prediction. In addition, according to the MPEG-2 standard, mutually different quantization matrices can be set for luminance data and chrominance data.
FIG. 1 shows a configuration for a conventional moving picture encoder 100, which includes an input picture memory 101, a subtracting section 102, an orthogonal transformation section 103, a quantization section 104, a variable-length encoding section 105, an inverse quantization section 106, an inverse orthogonal transformation section 107, an adding section 108, a reference picture memory 109, a motion detection/motion compensation section 110 and a rate control section 111.
The quantization operation using the quantity matrix is performed by the quantization section 104. First, the quantization section 104 multiplies the respective coefficient values of the quantization matrix by a quantization scale, thereby calculating quantization parameters. The quantization scale is generated by the rate control section 111 based on a target bit rate in encoding and the code size of the encoded picture data that has been generated by the variable-length encoding section 105.
Then, the quantization section 104 quantizes the respective frequency component values of the orthogonal transformation coefficient, which has been supplied from the orthogonal transformation section 103, with their associated quantization parameters, and outputs the results as quantized values. The quantization operation processing includes, as a main processing step, dividing the orthogonal transformation coefficient by the quantization parameter. Next, the variable-length encoding section 105 encodes each of those quantized values and outputs the result. In this manner, encoded picture data can be obtained.
The MPEG-2 standard defines standard quantization matrices that are used unless otherwise specified (which will be referred to herein as “default matrices”) as shown in FIGS. 4(a) and 4(b). For example, in the quantization matrix for intra coding shown in FIG. 4(a), the closer to the lower right corner of the matrix an element is, the greater the weight of that element is. As a result, a high-frequency component of the orthogonal transformation coefficient has low quantization precision. In the default matrix for non-intra coding shown in FIG. 4(b), on the other hand, all elements have the same value and there is no difference between their weights. That is why all high-frequency components of the orthogonal transformation coefficient have the same degree of quantization precision. In other words, the intra coding matrix has such frequency bit allocation characteristic that allocates a relatively large number of bits to low-frequency components, whereas the non-intra coding matrix has such frequency bit allocation characteristic that allocates the same number of bits equally to all frequency components.
Also, in the prior art, other quantization matrices, not just the examples described above, have also been used as shown in FIGS. 5(a) and 5(b). The matrices shown in FIGS. 5(a) and 5(b) are used for the last model of the MPEG-2 standard being set and are also called “TM5 quantization matrices”. As is clear from FIGS. 5(a) and 5(b), in each of these matrices, the coefficient values are weighted so as to decrease the quantity precision of high frequency components. That is why a relatively large number of bits are allocated to low-frequency components.
Supposing their quantization scales during the non-intra coding are the same, the quantized value of a high-frequency component that has been quantized using the TM5 quantization matrix becomes smaller than that of a high-frequency component that has been quantized with the MPEG-2 default matrix. As a result, the rate of the code generated can be decreased. This is because a greater amount of high-frequency component information has been cut down in the former case.
The MPEG-2 standard defines two types of quantization scales. FIG. 2 shows a quantization scale allocation table. Type 0 (qscale_type=0) and Type 1 (qscale_type=1) are defined for each of the quantization scale codes (quantiser_scale_code) #1 through #31 shown on the left column, and one of the two values is allocated as a quantization scale.
The rate control section 111 determines the value of the quantization scale by the target bit rate and the generated code size of the encoded picture data. If the quantization scale is the same, the degree of quantization precision of the frequency component, i.e., the bit allocation characteristic within the frequency space, is determined by the magnitude of the coefficient value of the quantization matrix.
As can be seen easily from the foregoing description, the quantization matrix and the quantization scale have significant influence on the quantization value. For example, Patent Document No. 1 discloses a technique for updating a quantization matrix on a screen basis according to the quantization scale.                Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 2001-78194        